Solid carbon dioxide blast cleaning is used in place of abrasive blasting systems and other blast cleaning systems to remove paint or other coatings/contaminants from surface areas. Most blast cleaning systems generate additional waste material which must be removed after the blast cleaning operation has been completed. In sandblasting, for example, sand is used as the blasting materials and a residual of sand is left around the area that has been blast cleaned. Using a material, such as solid carbon dioxide, in blast cleaning operation is advantageous because no residual blasting material remains, since the solid carbon dioxide sublimates to become gaseous carbon dioxide upon impacting the surface or warming. For this reason solid carbon dioxide blast cleaning is the preferred method of cleaning surfaces in certain environments where removal of the residual is difficult or impossible.
An example of carbon dioxide blast cleaning system is shown in U.S. Pat. No. 4,617,064, issued Oct. 14, 1986, to Moore.
Currently available commercial systems commonly have several standard components some of which are generally located on a large truck which is moved adjacent to the blast cleaning area and along with other components that are located at the blast site. Components located at the blast site are connected to the components carried by the truck through various flexible hoses and electric cables. In such systems, the truck typically carries a portable carbon dioxide vessel and other necessary equipment and machinery. The small portable carbon dioxide vessel includes an air compressor, diesel or electric generator for power supply, pelletizer with air dryer and feed system, and accompanying high pressure hose equipment. A large external carbon dioxide storage vessel (supply) is employed in such systems and is normally six (6) tons or greater in capacity. Since the rate of carbon dioxide usually varies between 500 pounds per hour to 1500 pounds per hour, the large external carbon dioxide storage vessel, which is supplying the smaller portable carbon dioxide vessel, may require filling more than once per day.
The air compressor employed is commonly a screw-type, having a rating of air flow at a range up to 500 cubic feed per minute at maximum pressures of around 250 PSI . An external power supply is required and a power supply of at least 70 amps and 220/460 volts is commonly utilized. Such external power is normally supplied by a portable generator located on the truck.
Located remotely therefrom at the blast site in such systems are a portable vessel containing liquid carbon dioxide, pelletizer, an air dryer, and a blasting gun having a nozzle to direct the pellets. A portable carbon dioxide vessel, normally holding approximately two tons, is filled from a large carbon dioxide storage vessel on the truck. The portable carbon dioxide vessel is adapted to be wheeled or otherwise moved to the blast site when pelletizing equipment is utilized to turn the liquid carbon dioxide into small carbon dioxide pellets. The pelletizing equipment normally has a typical capacity rate of around 200-500 pounds per hour of dry ice production. The pelletizer is operated by an electric power source through cable and flexible compressed air lines, as referred to hereinbefore, from a source of power supply and an air compressor mounted on the truck. Once pellets are made as stated, the same are delivered to a blasting gun attached to the pelletizer and driven by compressed air toward the surface to be cleaned.
The design of the pelletizer is well known in the art. A good description of the pelletizer is contained in the U.S. Pat. No. 4,617,064 issued Oct. 14, 1986 to Moore. Disclosure of this patent is hereby incorporated by reference. As stated above, a large liquid carbon dioxide storage tank is carried on the truck but said tank could also contain liquid air or other liquifiable gas which when vaporized can produce high pressure propellants.
Compressed air is carried from the compressor mounted on the truck by the flexible hose or cable to the blasting gun area after first passing through an air dryer normally located at the blasting site. The air dryer operates to lower the dew point of the compressed air down to -40 degrees Fahrenheit, to prevent water vapor from causing problems during the blasting process.
The above described currently available system has several inherent disadvantages. First, a multiplicity of lines, both air and electrical, must be run from the truck located outwardly of the blast area.
Secondly, available pressure from a conventional air compressor is limited to 250 pounds per square inch. The use of such commercial air compressors is not only difficult in operation but expensive.
Thirdly, the system ties the pelletizing machinery directly to the blast mechanism at the blasting site creating problems due to space limitations at the blasting site and requires that the components act as one unit rather than independently of one another.
Further, in the commercially available systems discussed hereinbefore, reduction of the moisture level of the incoming air down to a dew point of about -40 degrees Fahrenheit is necessary.
Another example of the prior art carbon dioxide blast cleaning system is U.S. Pat. No. 4,389,820 to Fong et al. The latter patent provides for the introduction of particles of carbon dioxide into a lower pressure transport gas flow. The gas flow with the paticles is then delivered to a nozzle to be accelerated and directed against the surface being cleaned. Fong's disclosure specifically states that the propellant gas temperatures are in the range of 100 degrees to 275 degrees Fahrenheit (F). The elevated temperature is essential to prevent freezing of the system (see Column 5, Line 65 to Column 6, Line 5). When the dry ice particles, having a temperature of approximately -109 degrees F., come into contact with the elevated gas flow temperature above 100 degrees Fahrenheit, the pellets will immediately begin to sublimate. The resulting reduction in the mass of dry ice particles decreases the efficiency of the blasting system.
The object of the present invention is to produce a carbon dioxide blast cleaning system and method of using the system in which carbon dioxide pellets are instantly available and are located at the blast site for instant use.
A further object of the invention is to produce a carbon dioxide blast cleaning system and method of using the system which is inexpensive in manufacture, being composed of fewer parts, and highly efficient in operation.
Another object of the invention is to provide a carbon dioxide blast cleaning system and method of using the system which eliminates the multiplicity of components located at a considerable distance from the blast site in the blasting operation as in the prior art systems.
A still further object of the present invention is to provide a carbon dioxide blast cleaning system and method of operating the system wherein the temperature of the propelling gas is low enough to prevent premature sublimation of the carbon dioxide pellets.
Still another object of the present invention is to provide a carbon dioxide blast cleaning system and method operating the system wherein the entire system is portable and easily moved and set up at a blasting site.
Other objects of the invention and the invention itself, will become apparent from purview of the appended description in which reference is made to the accompanying drawings.